1. Field of the Invention
The present invention relates to a valve operating mechanism of the SOHC (single overhead camshaft) type for an internal combustion engine.
2. Description of the Relevant Art
It is known in valve operating mechanisms of the SOHC type that rocker arms for operating intake and exhaust valves are supported on a common rocker arm shaft. Japanese Laid-Open Patent Publication No. 60-209612 and Japanese Laid-Open Utility Model Publication No. 61-152706 disclose SOHC-type valve operating mechanisms in which a camshaft is disposed above a rocker arm shaft, ignition spark plugs are disposed centrally above respective combustion chambers for increased combustion efficiency, and four valves are provided for each cylinder.
With the camshaft located above the rocker arm shaft, the rocker arms can be reduced in size and the rotational speed of the engine can be increased.
The centrally located spark plugs, which, are effective in knocking prevention, were previously installed through plug insertion pipes which were fitted and sealed in the cylinder head and a head cover. However, use of such additional plug insertion pipes results in an increased number of components required. Since the upper and lower ends of the plug insertion pipes have to be sealed, the number of steps for assembling the valve operating mechanism is large.